1. Field of the Invention
The present invention relates to a plasma processing method and a plasma processing apparatus.
2. Description of the Related Art
In general, in a manufacturing step of a semiconductor device, a semiconductor wafer is subjected to a CVD process, an etching process, a sputtering process, etc. A plasma processing apparatus is used for these processes. For example, in a magnetron RIE apparatus which is an example of the plasma processing apparatus, a high-frequency electric power is applied to a processing chamber which is hermetically sealed, and a plasma is produced therein. In a plasma atmosphere, a semiconductor wafer, etc., which is an object to be processed mounted in the processing chamber, is subjected to an etching process. In this case, in order to increase a plasma density and enhance an etching rate, a high-frequency electric power of, e.g. 13.56 MHz is generally applied by an AC power supply.
However, in the above magnetron RIE apparatus, for example, if such a high-frequency power of a single frequency is applied, electrons with high mobility are non-uniformly accumulated on the surface of the semiconductor surface and a gradient of potential occurs. Consequently, the device may be destroyed by a charge-up. Thus, means for preventing the charge-up is required. Such means must prevent the charge-up without lowering the etching rate.
In addition, if the high-frequency power of the single frequency is applied and, for example, CF.sub.4 gas is used as a processing gas to selectively etch away a silicon oxide film formed on a silicon substrate, the selection ratio decreases. The reason for this is that the density of the plasma is high and consequently the energy absorbed by the plasma increases and dissociation of gas molecules progresses greatly. Where the processing gas is CF.sub.4, the dissociation progresses to the last phase, and most of radical components become F* (fluorine radical). Consequently, etching durability of a resist deteriorates considerably, resulting in a decrease of selection ratio. If the dissociation of gas molecules has progressed to the last phase, a great amount of C occurs and adheres to the surface of the silicon substrate. Moreover, where an AC high-frequency electric power having a single sine waveform is used, it is difficult to control dissociation of gas molecules.
For example, in the case where an etching process is performed as a plasma process, it is known that the lower the pressure (process pressure) of a gas atmosphere in the processing chamber, the more suitably the fine processing can be performed. For example, when a 4 MDRAM is formed, it is necessary to perform a fine process with a line width of about 0.8 .mu.m. In this case, it is necessary to set the process pressure at about 1.7 Torr. Moreover, for example, when a 16 MDRAM is manufactured, it is necessary to perform fine processing with a line width of about 0.5 .mu.m, and in this case the process pressure needs to be set at about 0.25 Torr.
Normally, when a processing gas is introduced into the processing chamber, the gas pressure in gas introducing means is made slightly higher than that in the processing chamber owing to a resistance applied by a gas introducing pipe, etc. However, if the process pressure decreases to about 0.25 Torr, as stated above, the gas pressure in the gas introducing means decreases considerably in accordance with the decrease of the process pressure. Thus, a plasma discharge occurs not only in the processing chamber but also in the gas introducing chamber. If the plasma discharge occurs in the gas introducing means, the material of the gas introducing means is hit with active species in the plasma and particles are produced. These particles flow into the processing chamber and adhere to the semiconductor wafer.